The invention relates to a process for incinerating waste products on a fired grate, in which process the waste products are first dried and ignited, whereupon the main combustion process is carried out and thereafter the resultant slag is discharged.
In past years the fuel refuse or waste has constantly changed in composition to the extent that the proportion of volatile constituents has increased, which is essentially introduced via plastics. In parallel to this, the supply of carbon for combustion directly on the fired grate has decreased. As a result of this development the temperature of the fuel bed and the slag bed on the fired grate has also decreased.
Both phenomena make it more difficult to achieve very good waste slag burnup quality.
By means of certain measures during firing on the fired grate, attempts have already been made to increase again the bed temperature on the fired grate. The most important of these measures are reducing the excess air in the primary combustion on the fired grate, intensifying the fuel bed circulation on the fired grate, for example by increasing the grate stroke rate per unit time, and increasing the primary air temperature. However, there are narrow limits set to a reduction in the excess primary air, since an air feed which is no longer sufficient locally increases the proportion of unburnt solids. Increasing the grate stroke rate, if there is no desire to increase the transport rate of the material stream on the fired grate particularly greatly, is only possible using a reciprocating grate and can lead to an unwanted increase in dust discharge with the combustion exhaust gases. Increasing the primary air temperature, if fuel fractions having particularly high heating values are charged onto the grate, can always lead to unwanted vigorous ignition reactions, resulting in combustion, sometimes uncontrolled. These measures which have been employed previously to achieve improved slag quality have therefore only led to partial success.
It is an object of the invention to provide a process by means of which the combustion processes on the fired grate can be controlled in such a manner that the quality of the resultant slag is improved with respect to its reuse or ability to be landfilled, without the abovementioned disadvantages.
This object is achieved according to the invention starting from a process of the type described at the outset by virtue of the fact that, at the end of the main combustion process, where burnable fractions are still present in addition to slag components which are already forming, the burnup rate or burnup intensity is retarded in selectable sequential time sections and is increased in the intermediate time sections.
In the course of the combustion process, the combustion bed, at the end of the main combustion process, passes into a state in which the degree of burnup of the fuel bed is substantially complete and the slag which forms begins to cool. This transition takes place fluidly and frequently changes its position on the fired grate depending on the fuel quality. In this region, zones having wastes which are still to be burnt up and zones having slag which is already beginning to cool exist next to one another in the manner of islands. Since the surface of the fired grate is no longer uniformly covered there with fuel which is still burning, the fuel bed temperature already begins to fall and the slag resulting there can no longer achieve the highest possible quality.
By means of the inventive measure, that is to say the decrease in burnup rate or burnup intensity at times in this advanced state of the main combustion process, the carbon present in the fuel burns slower there and thus during this period in the transition region to slag formation more carbon collects than if a retardation in the burnup rate were not to occur. Thus for the following time section, in which the burnup rate or burnup intensity increases again, sufficient carbon is present in order to increase the combustion bed temperature to the extent that the slag forming in this process has the required quality. The combustion bed temperatures achieved by this procedure are higher than the combustion bed temperatures which can be achieved in the case of uniform combustion sequence operation, because of the deficient carbon supply in the transition region of the fired grate.
By means of the inventive procedure, the combustion bed temperature at the end of the main combustion process is still as high as possible, as stable as possible and has the most even distribution possible over the area on which the main combustion process decreases in intensity. These are the most essential preconditions which must be sought after in order to achieve good slag quality and which are achieved by the inventive process, since, owing to the repeated variation in burnup rate or burnup intensity, increased accumulation of carbon repeatedly occurs owing to the decrease in burnup rate or burnup intensity and a temperature increase following this process is achieved by increasing the burnup rate or burnup intensity in the region in which, hitherto, owing to the altered quality of the fuel, the combustion bed temperature was no longer sufficient to form the desired slag quality. Repeated enrichment of carbon-rich combustion material thus occurs in the region which previously featured too low a combustion bed temperature, in order by burning this repeatedly increased carbon content to increase the combustion bed temperature in the region of slag formation and also to stabilize it and to distribute it evenly.
A preferred measure according to the invention is that the retardation and acceleration of the burnup rate or burnup intensity at the end of the main combustion process are achieved by repeated variation of the primary air rate in the respective time sections in the burnup stage critical for slag quality.
The primary air rate can be varied by decreasing the primary air rate below a standard measure and subsequently increasing the primary air rate to the standard measure. Although, to increase the burnup rate, the primary air rate is preferably increased again to the usual standard measure, the burnup rate or burnup intensity is increased because of the accumulated increased carbon content. Increasing the primary air rate above the standard measure is not required in most cases here. A particularly expedient process sequence is characterized in that the time sections having a quantitatively decreased primary air rate and the time sections having a primary air rate which is standard for this fired grate region alternate constantly in a selectable ratio to one another. Preferably, the two time sections are in the ratio of 1:1.
The sought-after carbon enrichment in the fuel bed at the end of the main combustion process depends not only on the reduction in burnup intensity or burnup rate and thus, for example, on the reduction in primary air rate, but also significantly on the grate movement, that is to say on the fuel advance and the fuel transport rate. It is therefore advantageous if the time section having a decreased burnup intensity or reduced primary air rate is in a preselectable ratio to the number of double grate strokes. To influence the desired carbon enrichment, it is also advantageous if the number of double grate strokes per time section can be controlled.
In practice, it has proved to be advantageous if a time section of decreased burnup intensity or reduced primary air rate is 3 to 6 minutes. It is expedient if the reduced primary air rate is 50 to 70% of the standard primary air rate. An advantageous development of the invention is that the variation in primary air rate at the end of the main combustion process, based on the total combustion process, is quantitatively neutral, that is to say the total amount of primary air is not significantly changed by the inventive procedure compared with a previous standard operation.
The target of improving slag quality is also advantageously influenced by the primary air temperature being increased compared with the ambient air temperature. Preferably, the primary air temperature is controlled in the range from 110xc2x0 C. to 180xc2x0 C.
The burnup rate or burnup intensity can also be changed by varying the O2 content of the primary air. It is advantageous here if the O2 content of the primary air is varied with constant mass flow rate of the primary air.